virsh: add qemu-monitor-command --pretty

virsh: add qemu-monitor-command --pretty

I was using qemu-monitor-command during development, and found it quite
hard to use.  Compare the results of this patch on ease of reading:

$ virsh qemu-monitor-command          dom '{"execute":"query-version"}'
{"return":{"qemu":{"micro":1,"minor":12,"major":0},"package":"(qemu-kvm-0.12.1.2)"},"id":"libvirt-7683"}

$ virsh qemu-monitor-command --pretty dom '{"execute":"query-version"}'
{
    "return": {
        "qemu": {
            "micro": 1,
            "minor": 12,
            "major": 0
        },
        "package": "(qemu-kvm-0.12.1.2)"
    },
    "id": "libvirt-7674"
}

* tools/virsh-host.c (cmdQemuMonitorCommand): New option.
* tools/virsh.pod (qemu-monitor-command): Document it.